Method of ranking neighbour cells as candidates for an hand over

ABSTRACT

The present invention is related to cellular radio communication network and a method and apparatus for ranking a set of neighbor cells associated with a first cell in the cellular radio communication network. For each cell member in said neighbor cell set, handover statistics reflecting how frequent said cell member is involved in handovers aiming at modifying active sets including at least said first cell to include said cell member are registered ( 301 ). Based at least in part on the registered handover statistics for the different cell members in the set of neighbor cells, the cell members of said set of neighbor cells are ranked ( 302 ). The invention also includes a method for controlling mobile station measurements wherein neighbor cells are selected for inclusion in a monitored set based on said ranking.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a cellular radio communicationnetwork and methods and an apparatus in the cellular radio communicationnetwork. More in particular, the invention relates to a method andapparatus for ranking of neighbor cell sets in the radio communicationnetwork and a radio communication network including said apparatus. Theinvention also includes a method of controlling mobile stationmeasurements.

DESCRIPTION OF RELATED ART

[0002] In a typical cellular radio communication system, a geographicalarea served by a cellular radio communication network is divided intocell areas in which radio base stations, also sometimes referred to asbase transceiver stations, provide radio coverage to mobile stationsoperating in said cell areas. The mobile stations, which e.g. may beportable, pocket, hand-held or car mounted, enables mobile station users(mobile subscribers) to communicate voice, data and/or multimediainformation via the cellular radio communication network. Each radiobase station may be equipped to service one or more cells.

[0003] A mobile station may be assigned a radio communication channeldedicated for communication between the mobile station and the cellularnetwork e.g. when receiving or making a phone call. In traditionalcellular radio communication systems, such as e.g. Global System forMobile communication (GSM), Personal Digital Cellular (PDC) and DigitalAdvanced Mobile Phone System (DAMPS), the dedicated radio communicationchannel is served by a single cell referred to as the serving cell.During the phone call it may become necessary to serve the call usinganother cell e.g. due to the mobile station moving into an other cell. Anew dedicated radio communication channel is then established in theother cell for serving the phone call, while the old dedicated radiocommunication channel is released. This process of changing dedicatedradio communication channels during an ongoing call is referred to ashandoff or handover. Due to there being a short interruption ofcommunication as the mobile station switches from one dedicated radiocommunication channel to another, this type of handover is sometimesreferred to as hard handover or “break before make”.

[0004] In more recent cellular radio communication systems using socalled Code Division Multiple Access (CDMA) technology, e.g. IS-95 andthe UTRAN FDD mode (also referred to as Wideband CDMA), a dedicatedradio communication channel need not only be supported by a singleserving cell at each given moment of time, but may on the contrary besupported by several cells using so called macrodiversity which providesincreased radio transmission quality. The set of cells currently servingthe dedicated communication channel is referred to as the Active Set.Due to the possibility of using more than one cell to serve a dedicatedradio communication channel, these cellular radio communication systemsalso enables so called soft handover, sometimes referred to as “makebefore break”, wherein the mobile station continues to communicate withthe cellular network via the old cell at least until the dedicated radiochannel is established also in the new cell.

[0005] In order to support handover, both in traditional systems andsystems using CDMA, mobile stations are required to perform measurementson downlink transmissions, i.e. from the cellular network to the mobilestations, in both the serving cell/Active Set as well as neighboringcells to the serving cell/Active Set. The measurement results arereported back to the cellular network and are used for making decisionson which cell is best suited to serve the respective mobile station.

[0006] The mobile stations have limited capacity for performing thedownlink measurements, thus if a mobile station is ordered to performdownlink measurements on transmissions in too many cells, the quality,and thereby the reliability of the measurements reported to the network,is reduced and/or it takes longer time to produce measurement resultswhich e.g. may cause a loss of communication in a situation where theradio environment changes quickly such as when the mobile station turnsaround a street corner at high speed. On the other hand, if not allneighbor cells to the serving cell/Active Set are measured, there is arisk that the most suitable cell, i.e. the cell where communicationbetween the mobile station and cellular network can be maintained usingminimum radio transmission power, is never considered as a candidate forhandover. To be able to use minimum transmission power for maintainingcommunication is important since this way battery operated mobilestations can operate for longer periods of time without having to berecharged. To be able to use minimum transmission power is also veryimportant in so called interference limited systems, such as systemsbased on CDMA-technology, wherein using minimum transmission powercauses reduced interference levels which translates into improved systemcapacity.

[0007] Since when employing macrodiversity/soft handover, the Active Setserving a mobile station may include several cells, e.g. upto 6 cells inUTRAN FDD mode, which each has a plurality of neighbor cells, the aboveillustrated problem of the number of neighbor cells associated withcells in an active set exceeding the number of neighbor cells for whichmobile stations have adequate capacity to perform downlink measurementappears frequently in cellular communication systems employingCDMA-technology, in particular if such systems also provide thepossibility for handover to neighbor cells on other frequencies and/orneighbor cells based on other radio access technologies.

[0008] In current UMTS Radio Access Networks based on the FDD mode, aknown way of handling a situation where there are too many neighborcells associated with an Active Set supporting communication between amobile station and the radio access network, is to order the mobilestation to perform measurements on an arbitrary selection of saidneighbor cells.

[0009] Published international patent application WO 98/36588 describesa method and apparatus in a code division multiple access (CDMA)communication system for merging pilot neighbor lists to form a NeighborList Update message for transmission to a mobile station operating inthe CDMA-system. The mobile station measures pilot signal strengths andphase delays received at the mobile station. The mobile stationcommunicates the signal strengths and phase delays of all pilot signalsin the so called Active Set and (optionally) Candidate Set to a MobileTelephone Switching Office (MTSO) in the CDMA-system. The MTSO retrievespredetermined neighbor lists of all Active/Candidate set pilots reportedby the mobile station and generates a neighbour list union as the unionof the predetermined neighbor lists. Any member in the neighbor listunion included in the Active/Candidate set reported by the mobilestation is removed and each remaining member, or base station, in theneighbor list union is operated on by a weighting function which takesinto account how often each member is included in the predeterminedneighbor lists of the reported Active/Candidate set pilots and thesignal strength of such pilots. The relative weight are then sorted andthe highest ranking members form a Neighbor List Update Message, up to apredetermined number of members.

[0010] U.S. Pat. No. 6,119,005 discloses a method and system forautomated determination of handoff neighbor lists for base stations in aCode Division Multiple Access (CDMA) system. Mobile subscriber unitsoperating in the CDMA-system produces Pilot Strength Measurement (PSM)data that is indicative of the relative signal strength measured at themobile subsriber units from a plurality of pilot channels transmitted bybase stations in the CDMA-system. The mobile subscriber units transmitthis data along with a list of viable pilot channel candidates asdetermined by the mobile subscriber units, to the base stations servingthe respective calls. For each base station in the CDMA-system, thesystem for automated determination of handoff neighbor list maintains adata structure that stores data indicative of the number of instancesthat a pilot channel is recommended as a viable candidate by mobilesubscriber units involved in calls served by the base station and thesum of power levels that were measured by the various mobile subscriberunits for these instances. This data is processed to determine a metricfor each pilot channel entry in the data structure data, said metricbeing a function of the number of instances that the pilot channel isrecommended multiplied by a weighting factor summed with the sum ofpower levels. Based on the determined metric values, the pilot channelentries are rank ordered and the resultant list is truncated to apredetermined number of entries forming an updated Neighbor List for thebase station at hand.

[0011] Published international patent application WO 01/17293 teaches amethod and an arrangement in a cellular radio communication network forpreparing a measurement order list for transmission to a mobile stationoperating in a first cell. According to the disclosed method, it isdetermined whether at least one cell among the neighbouring cells to thefirst cell is incapable of acting as a target for handoff of the mobilestation. If at least one cell incapable of acting as a target forhandoff is found, at least one control channel associated with such acell is omitted when generating said list by including control channelsassociated with neighbouring cells.

SUMMARY OF THE INVENTION

[0012] The problem dealt with by the present invention is to provideconditions for increasing the probability that the most suitable cellsare included in an Active Set supporting communication between acellular radio communication network and a mobile station.

[0013] The problem is solved essentially by a method of ranking sets ofneighbor cells based at least in part on how frequent each neighbor cellmember is involved in handovers and a method of controlling measurementswhich accounts for the ranking established using the inventive methodfor ranking. The invention also includes an apparatus implementing themethod of ranking and a cellular radio communication network includingsuch an apparatus.

[0014] More specifically, the problem is solved using methods accordingto claim 1 and 17, an apparatus according to claim 18 and a radiocommunication network according to claim 29.

[0015] An advantage afforded by the invention is that it providesconditions for increasing the probability that the most suitable cellsare included in an Active Set supporting communication between acellular radio communication network and a mobile station.

[0016] Another advantage offered by the invention is that it enablesinterference reductions in a cellular radio communication system.

[0017] Yet another advantage afforded by the invention is that, whenapplied in an interference limited system, such as a system based onCode Division Multiple Access technology, it enables increased systemcapacity.

[0018] The invention will now be described in more detail with referenceto exemplary embodiments thereof and also with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic view of a communication system including acellular radio communication network.

[0020]FIG. 2A-2C are views illustrating an example scenario of how cellsof the cellular radio communication network provide radio coverage in anarea surrounding a mobile station.

[0021]FIG. 3A is a flow chart illustrating a basic method according tothe invention for ranking.

[0022]FIG. 3B is a flow chart illustrating a basic method according tothe invention for controlling mobile station measurements.

[0023]FIG. 4 is a block diagram illustrating an example data structurefor storing neighbor cell data.

[0024]FIG. 5 is a flow chart illustrating a first exemplary methodaccording to the invention for ranking.

[0025]FIG. 6 is a flow chart illustrating a first exemplary methodaccording to the invention for controlling mobile station measurements.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026]FIG. 1 illustrates a non-limiting example of a communicationsystem SYS1 in which the present invention may be employed. Theexemplary communication system SYS1 illustrated in FIG. 1 is a UniversalMobile Telecommunication System (UMTS). The communication system SYS1includes a cellular radio communication network NET1, alternativelyreferred to as a Public Land Mobile Network (PLMN), and User Equipment(UE), alternatively referred to as mobile stations (MS).

[0027] The exemplary cellular radio communication network includes acore network CN1 and two access networks, a UMTS Terrestrial RadioAccess Network (UTRAN) RAN1 and a GSM Base Station System (BSS) BSS1.

[0028] The core network CN1 includes a Mobile services Switching Center(MSC) node MSC1 that provides circuit-switched services and a GeneralPacket Radio Service (GPRS) node SGSN1, sometimes referred to as aserving GPRS support node (SGSN), which is tailored to providepacket-switched type services.

[0029] The UMTS Terrestrial radio access network RAN1, referred to asradio access network RANL in the following, and the GSM Base StationSystem, referred to as base station system BSS1 in the following, bothprovide radio communication between the cellular radio communicationnetwork NET1 and mobile stations, such as mobile station MS1 in FIG. 1,but using different radio access technology. Thus the radio accessnetwork RAN1 uses the new Wideband Code Division Multiple Access (WCDMA)radio access technology, while the base station system uses GSM/EDGEradio access technology which are both well known to a person skilled inthe art.

[0030] The radio access network RAN1 includes one or more radio networkcontrollers (RNCs). For sake of simplicity, the radio access networkRAN1 of FIG. 1 is shown with only one radio network controller nodeRNC1. Each radio network controller is connected to and controls aplurality of radio base stations RBS1. The radio access network RAN1 isconnected to both core network nodes MSC1 and SGSN1 over a radio accessnetwork interface referred to as the Iu interface.

[0031] The base station system BSS1 includes one or more base stationcontrollers (BSCs). For sake of simplicity, the base station system BSS1of FIG. 1 is shown with only one base station controller node BSC1. Eachbase station controller is connected to and controls a plurality of basetransceiver stations (BTSs) BTS1. The base station system BSS1 isconnected to the mobile services switching center MSC1 over an interfacereferred to as the A interface while the base station system BSS1 isconnected to the serving GPRS support node SGSN1 over an interfacereferred to as the Gb interface.

[0032] As previously indicated, mobile stations, such as mobile stationMS1 shown in FIG. 1, may communicate with the cellular radiocommunication network NET1 either via the radio access network RAN1 orvia the base station system BSS1. Communication between mobile stationsand the base station system BSS1 occurs over a radio interface referredto as the Um interface while communication between mobile stations andthe radio access network RAN1 occurs over a radio interface referred toas the Uu interface. The details of the Um-interface are specified inthe 04- and 05-series of GSM Technical Specifications while the detailsof the Uu-interface are specified in the 24- and 25-series of UMTSTechnical Specifications.

[0033]FIG. 2A-2C illustrate an example scenario of how the exemplarycellular radio communication network NET1 provides radio coverage in angeographical area where the mobile station MS1 is currently located. Inthis example scenario, radio coverage is provided in the geographicalarea where the mobile station MS1 is located both by the radio accessnetwork RAN1 as well as the base station system BSS1. In this examplescenario, the radio access network RAN1 operates in the FrequencyDivision Duplex (FDD) mode of UTRAN and provides radio coverage usingcells assigned a first pair of duplex radio frequency (RF) carriers,i.e. an uplink RF carrier and a downlink RF carrier, as well as usingcells assigned a second pair of duplex RF carriers. Furthermore, in thisexample scenario, the base station system BSS1 provides radio coverageusing cells assigned uplink and downlink radio carrier frequencies inone of the GSM frequency bands, e.g. the 900 MHZ band.

[0034]FIG. 2A illustrates schematically cells C11-C22 of the radioaccess network RAN1 covering the area where the mobile station MS1 islocated using the first pair of duplex radio frequency carriers. Asillustrated the cells C11-C22 partly overlap each other and may be ofdifferent size as well as different shape.

[0035] In similar ways, FIG. 2B illustrates schematically cells C31-C43of the radio access network RAN1 covering the area where the mobilestation MS1 is located using the second pair of duplex radio frequencycarriers, and FIG. 2C illustrates schematically cells C51-C61 of thebase station system BSS1 covering the same area. In FIG. 2A and FIG. 2B,the geographical cell borders of cells C11 and C12 are illustrated asthin dashed lines.

[0036] As illustrated by FIG. 2A to 2C both cell C11 and cell C12 have aplurality of neighbor cells, i.e. cells which are providing radiocoverage in areas overlapping cell C11 and cell C12 respectively andcells which are providing radio coverage in areas adjacent to cells C11and C12 respectively. Thus, cells C12-C18 in FIG. 2A, cells C31-C38 andC43 in FIG. 2B as well as cells C51 and C54-C58 constitute a set ofneighbor cells associated with cell C11 while cells C11, C13, andC18-C22 in FIG. 2A, cells C31-C34 and C38-C42 in FIG. 2B as well ascells CS1-C54 and C58-C61 in FIG. 2C constitute a set of neighbor cellsassociated with cell C12.

[0037] In the context of the present example scenario, the mobilestation MS1 is assumed to be in a state of active communication with thecellular network NET1 using a dedicated radio communication channelsupported by an active set consisting of cells C11 and C12. It isfurther assumed that the mobile station MS1 is capable of communicatingboth according to the Um interface, i.e. using the GSM air interface, aswell as according to the Uu interface, i.e. using the UTRAN FDD mode airinterface.

[0038] As previously discussed, the mobile station MS1 need to performdownlink radio transmission measurements for the purpose of supportinghandover. Thus the radio access network RAN1 orders the mobile stationMS1 to perform measurements on downlink radio transmissions in the cellsof the active set, i.e. cells C11 and C12 in the present examplescenario, as well as in the cells of a so called monitored setcomprising neighbor cells to the cells in the active set, i.e. cellswhich are likely to become targets for handover in the future.

[0039] If the mobile station MS1 were to perform downlink measurementsfor cells C11 and C12 in the active set as well as all neighbor cells tocells C11-C12, the mobile station MS1 would need to perform measurementsfor all cells illustrated in FIG. 2A and FIG. 2B, i.e. a total of 36cells.

[0040] The current specifications for UTRAN FDD mode (see 3 GPPtechnical specification 25.331) allows a radio access network to ordermeasurement of downlink transmissions in upto 32 cells each of cellsoperating at the same downlink frequency as the cells of the active set(Intra-frequency measurements), cells operating at downlink frequenciesthat differ from the frequency of the cells in the active set(Inter-frequency measurements) and cells using another Radio AccessTechnology (Inter-RAT measurements). Thus a mobile station may beordered to perform downlink transmission measurements for a total of 96cells.

[0041] The capacity for performing downlink transmission measurementsmay vary from mobile station to mobile station, but may in manyinstances be significantly less than 32 cells each for Intra-frequency,Inter-Frequency and Inter-RAT measurements, e.g. in the order of 8-10cells for each different category, i.e. a total of 24-30 cells. If amobile station is ordered to perform too many downlink transmissionmeasurements, the rate at which the mobile station will be able toreport measurement results to the radio access network will be severelyreduced and may thus result in a significant delay before handover tochange a current active set to a more suitable active set can beperformed. This in turn causes increased interference levels due toradio communication between the mobile station and the radio accessnetwork occurring using unnecessarily high transmission power levels.The delayed handover may also result in a loss of communication in asituation where the radio environment changes very quickly. In order toeliminate or at least reduce the risk for ordering a mobile station toperform downlink transmission measurements in excess of its capacity,configuration parameters may be provided in a radio access networkenabling operation and maintenance personnel to control the maximumnumber of downlink transmission measurements in each category that theradio access network is allowed to order mobile stations to perform.

[0042] Assuming in the present example scenario that configurationparameters in the radio access network RNC1 specifies a maximum of 8cells each for Intra-frequency, Inter-Frequency and Inter-RAT downlinktransmission measurements, i.e. a total of 24 cells, the mobile stationMS1 can not be ordered to perform downlink transmission measurements forall neighbor cells to cells C11 and C12 in the current active set.

[0043] In current UMTS Radio Access Networks, a known way of handling asituation where there are too many neighbor cells associated with anActive Set supporting communication between a mobile station and theradio access network, is to order the mobile station to performmeasurements on an arbitrary selection of said neighbor cells. However,since only an arbitrary selection of all neighbor cells are measured,there is a significant risk that the most suitable cell, i.e. the cellwhere communication between the mobile station and the radio accessnetwork can be maintained using minimum radio transmission power levels,is never considered as a candidate for handover. Not using the mostsuitable cell for radio communication between the radio access networkand the mobile station means that said radio communication occurs usingunnecessarily high transmission power levels, which causes increasedinterference. The increased interference in turn translates to reducedsystem capacity.

[0044] The present invention deals with situations as the oneillustrated in the example scenario above by providing conditions forincreasing the probability that the most suitable cells are included inan Active Set supporting communication between a cellular radiocommunication network and a mobile station when the capacity forperforming downlink radio transmission measurements is limited.

[0045] The present invention solves the above stated problem by a methodfor ranking neighbor cells and a method for controlling measurementsperformed by a mobile station which accounts for the established rankingof neighbor cells. The invention also includes an apparatus for rankingneighbor cells and a cellular radio communication network including suchan apparatus.

[0046]FIG. 3A illustrates a basic method according to the invention forranking a set of neighbor cells registered in a cellular radiocommunication network as neighbor cells to a first cell of the cellularradio communication network.

[0047] For each cell member in said neighbor cell set, handoverstatistics on how frequent said cell member is involved in handoversaiming at modifying active sets including at least said first cell toinclude said cell member is registered at step 301.

[0048] At step 302 ranking of said set of neighbor cells is performedaccording to a predetermined rule based at least in part on theregistered handover statistics for the different cell members in the setof neighbor cells.

[0049]FIG. 3B illustrates a basic method according to the invention forcontrolling measurements performed by a mobile station operating withinan area served by a cellular radio communication network.

[0050] At step 311 a current active set of one or more cells providingradio communication between the mobile station and the cellular radiocommunication network is determined.

[0051] At step 312 neighbor cells to the cells in the current active setare selected for inclusion in a monitored set of cells. The selection ofneighbor cells for inclusion in the monitored set is based at least inpart on ranking of the neighbor cells of the current active setestablished using the inventive method for ranking neighbor cell sets.

[0052] At step 313 the cellular radio communication network orders themobile station to perform measurements on downlink transmissions in thecells of the monitored set.

[0053]FIGS. 4 and 5 illustrates a first exemplary embodiment of theinvention for ranking implemented in the radio access network RAN1 inFIG. 1.

[0054]FIG. 4 illustrates an exemplary data structure in the radionetwork controller RNC1 for storing neighbor cell data. In FIG. 4, theexemplary data structure 400 is used for storing data associated withthe neighbor cells of cell C11 in FIG. 2A.

[0055] The data structure 400 includes three tables:

[0056] a first table 401 for storing data associated with neighbor cellsusing the same first pair of duplex radio frequency carriers as cellC11, i.e. cells C12-C18 in FIG. 2A;

[0057] a second table 402 for storing data associated with neighborcells using the second pair of duplex radio frequency carriers, i.e.cells C31-C38 and C43 in FIG. 2B;

[0058] a third table 403 for storing data associated with neighbor cellsof the base station system BSS1, i.e. cells C51 and C54-C58.

[0059] The three tables 401-403 represent a division of the total set ofneighbor cells to cell C11 into three subset of neighbor cells. In thefollowing the subset represented by table 401 will be referred to asintra-frequency neighbors, the subset represented by table 402 will bereferred to as inter-frequency neighbors and the subset represented bytable 403 will be referred to as inter-RAT neighbors.

[0060] For each neighbor cell registered in the tables 401-403 a cellidentity field 404, a rank field 405 and a usage counter field 406 areprovided.

[0061] The cell identity field 404 is used to identify each neighborcell and is also used as a key to access additional information abouteach neighbor cell that is necessary for ordering downlink transmissionpower measurements. This additional information about each neighbor cellis stored in other data structures in the radio network controller RNC1and includes scrambling codes for each intra-frequency neighbor cellregistered in the first table 401, scrambling code and carrier frequencyfor each inter-frequency neighbor cell registered in the second table402 and Broadcast Control Channel (BCCH) carrier frequency and BaseTransceiver Station Identity Code (BSIC) for each inter-RAT neighborcell registered in the third table 403.

[0062] The rank field 405 specifies the rank assigned to respectiveneighbor cell member.

[0063] The usage counter field 406 is used to register handoverstatistics for the respective neighbor cell member.

[0064]FIG. 5 illustrates processing steps performed in the radio networkcontroller RNC1 in accordance with the first exemplary embodiment of theinvention for ranking the members of the set of neighbor cellsassociated with cell C11.

[0065] At step 501 an initial arbitrary ranking order of all cellmembers of the neighbor cell set associated with cell C11 isestablished. This arbitrary order may e.g. correspond to the order bywhich neighbor cells have been added to the respective tables 401-403.The initial ranking, and all subsequent rankings, are performed on eachtable 401-403 separately, i.e. each subset of neighbor cells is rankedseparately. The rank field 405 of all neighbor cells registered intables 401-403 are set according to the initial ranking order.

[0066] At step 502 a statistics gathering period is initiated by settinga timer to a selected time period value and initiating all usage counterfields 406 in the tables 401-403 to a count of zero. The timer value isdefined by a configuration parameter enabling operation and maintenancepersonnel to control the statistics gathering period length. Typicallythe timer value would be selected as e.g. 24 hours.

[0067] During the statistics gathering period, processing is controlledby two types of events, handovers and timer expiry. The radio networkcontroller RNC1 awaits these events at step 503.

[0068] Each time a handover event (an alternative HANDOVER at step 503)is detected at step 503, the radio network controller RNC1 determines atstep 504 whether the handover occurred for an original active setincluding at least cell C11 and also whether the handover was a handovercausing one of the neighbor cells of C11 to become part of the newactive set. If both these conditions are fulfilled (an alternative YESat step 504), processing continues at step 505 where the usage counter406 associated with the neighbor cell member included in the new activeset is incremented by one. Processing then returns to step 503.Otherwise (an alternative NO at step 504), i.e. when not both conditionsof step 504 are fulfilled, processing returns directly to step 503.

[0069] Upon timer expiry (an alternative TIMER EXPIRY at step 503), i.e.when the statistics gathering period has come to an end, the radionetwork controller RNC1 proceeds at step 506 by ranking the neighborcells to cell C11 based on the registered handover statistics, i.e. thevalue of the usage counter fields 406 in tables 401-403, for eachneighbor cell to cell C11.

[0070] As previously indicated, ranking is performed separately for eachtable 401-403 and is performed as follows.

[0071] A tentative ranking is first determined based strictly on thenumber of times each cell member has been involved in handovers asdetermined by the respective usage counters 406. For cell membersinvolved in the same number of handovers, the relative ranking orderestablished last time step 506 was performed is maintained.

[0072] The tentative ranking order is then adjusted to arrive at a newranking order by assigning the highest rank to the cell member that wasassigned the lowest rank according to the tentative ranking and rankingall other cell members one step lower than according to the tentativeranking.

[0073] Processing then continues at step 502, where a new statisticsgathering period is initiated and processing continues as describedabove.

[0074] In the first exemplary embodiment of the invention, one or morecontrol processors CP1 in the radio network controller RNC1 of FIG. 1are programmed to perform all the processing associated with the methodillustrated in FIG. 5. Thus the radio network controller RNC1 can beregarded as an apparatus for ranking the neighbor set cell members andthe control processors CP1 functions both as handover statisticsgathering circuitry as well as ranking circuitry for ranking theneighbor set cell members based on the gathered handover statistics.

[0075]FIG. 6 illustrates a first exemplary embodiment of the inventionfor controlling downlink transmission measurements performed by a mobilestation, e.g. mobile station MS1, operating in the area served by radioaccess network RAN1 in FIG. 1. The method is illustrated by means of thepreviously used example scenario, wherein the mobile station MS1 is incommunication with the radio access network RAN1 using a current activeset consisting of cells C11 and C12.

[0076] At step 601 the current active set, e.g. cells C11 and C12,providing radio communication between the mobile station MS1 and theradio access network RAN1 is determined.

[0077] At step 602 the number of neighbor cells from each differentneighbor cell subset associated with the cells in the active set, i.e.cells C11 and C12, to be included in a monitored set are determined. Foreach different subset category, i.e. Intra-frequency, Inter-frequencyand Inter-RAT neighbor cells respectively, each cell in the active setis allowed, as far as possible, to contribute with an equal number ofneighbor cells. Since, as previously described, we assume that in thepresent example scenario, configuration parameters of the radio accessnetwork RAN1 specifies a maximum of 8 cells each for Intra-frequency,Inter-frequency and Inter-RAT downlink measurements, and the currentactive set comprises cells C11 and C12, a total of 6 Intra-frequencyneighbor cells, 8 Inter-frequency neighbor cells and 8 Inter-RATneighbor cells are allowed in the monitored set. Thus, the active setcells C11 and C12 may each contribute with 3 Intra-frequency, 4Inter-frequency and 4 Inter-RAT neighbor cells to the monitored set.

[0078] At step 603 neighbor cells to the cells in the current active setare selected for inclusion in the monitored set of cells. The selectionof neighbor cells is based on ranking of the sets of neighbor cellsassociated with cell C11 and C12 respectively which are establishedusing the above described exemplary first embodiment of the inventionfor ranking. The highest ranked cells in each neighbor cell subsetassociated with the active set cells C11 and C12 are selected. Thus,among the neighbor cells associated with cell C11, the cell membersranked as the top three Intra-Frequency neighbor cells, the cell membersranked as the top four Inter-Frequency neighbor cells and the cellmembers ranked as the top four Inter-RAT neighbor cells, are selectedfor inclusion in the monitored set. Similarily, among the neighbor cellsassociated with cell C12, the cell members ranked as the top threeIntra-Frequency neighbor cells, the cell members ranked as the top fourInter-Frequency neighbor cells and the cell members ranked as the topfour Inter-RAT neighbor cells, are selected for inclusion in themonitored set. In the event that a neighbor cell associated with bothcells C11 and C12, i.e. a common neighbor cell, is selected twice, i.e.both as a highly ranked neighbor cell to C11 as well as a highly rankedneighbor cell to C12, an additional neighbor cell to either cell C11 orcell C12 of the same subset category as the common neighbor cell may beselected. Preferably this additional cell is selected such that, as faras possible, the monitored set includes, for each different category ofneighbor cells, an equal number of neighbor cells associated with cellsC11 and C12 respectively.

[0079] At step 604 the radio access network RAN1 orders the mobilestation MS1 to perform measurements on downlink transmissions in thecells of the monitored set.

[0080] Apart from the above disclosed exemplary first embodiments ofmethods according to the invention for ranking neighbor cells andcontrolling mobile station down link measurements, there are severalways of providing rearrangements, modifications and substitutions of thefirst embodiments resulting in additional embodiments of the invention.

[0081] In the example scenario illustrated in FIG. 2A-2C, GSM/EDGE radioaccess technology is used in cells C51-C54 defined as neighbor cells tocells C11 and C12 in which UTRAN FDD mode is used. It is of coursepossible for a first cell in an UMTS radio access network, or any othertype of radio access network, to have neighbor cells using one or moreother radio access technologies than that used in the first cell. Thus,cells C11 and C12 in FIG. 2A could instead of or in addition to cellsC51-C54 in which GSM/EDGE radio access technology is used, have neighborcells in which e.g. Personal Digital Cellular (PDC), Digital AdvancedMobile Phone System (DAMPS) or CDMA2000 radio access technology is used.

[0082] Instead of defining the statistics gathering period as lastingfor a predetermined time period, the statistics gathering period couldalternatively be defined as lasting until the total number of incrementsof all usage counters associated with a set of neighbor cells to a firstcell exceeds a predetermined threshold. Thus, dependent on how frequenthandover occurs, the statistics gathering period may last for a longeror shorter time period. The statistics gathering period length couldalso be defined based on a combination of an absolute time period andthe total number of increments of the usage counters associated with theset of neighbor cells. Thus, the statistics gathering period could e.g.be defined as lasting a certain minimum period of time but not endingbefore a certain total number of increments of the usage countersassociated with the set of neighbor cells exceeds a predeterminedthreshold value.

[0083] Instead of only including successful handovers in the handoverstatistics gathered for each cell member, i.e. only incrementing theusage counter associated with a cell member when indeed establishment ofcommunication in the cell member was successful, handover attempts,regardless of whether the handover was successful or not, could beincluded in the handover statistics for each cell member.

[0084] Instead of organizing the set of neighbor cells of a first cellinto at least two subsets and ranking each subset separately, the totalset of neighbor cells associated with the first cell could be ranked.Ranking the total set of neighbor cells to a cell in a single rankingorder would typically be performed in a situation where e.g. there areno Inter-frequency or Intra-RAT neighbors to the first cell. Ranking thetotal set of neighbor cells in a single ranking order could however alsobe performed in a situation where there are different categories ofneighbor cells.

[0085] Different rules can be applied when ranking the members of aneighbor cell set of a first cell based on handover statisticsreflecting how frequent each cell member is involved in handovers aimingat modifying active sets including at least the first cell to includesaid cell member. Typically however, cell members for which theregistered handover statistics indicate frequent involvement inhandovers are generally ranked higher than cell members for which theregistered handover statistics indicate less frequent involvement inhandovers. When performing repeated cycles of gathering handoverstatistics and ranking the cell members based on said statistics, aprobing mechanism may be implemented by, at least in some of therepeated ranking instances, ranking at least a first cell member higherthan at least one other cell member more frequently involved inhandovers than the at least first cell member. The probing mechanism isa way of offsetting the bias caused by the fact that neighbor cellswhich are never or only seldom included in monitored sets, stand alesser chance of being considered for handover than neighbor cells thatare often selected for inclusion. The first exemplary embodiment of amethod for ranking disclosed above, provides one example of such aprobing mechanism, wherein for each ranking instance, the cell memberassigned the lowest rank in the tentative ranking, i.e. the cell memberwhich according to the registered handover statistics is leastfrequently involved in handovers, is assigned the highest rank.

[0086] In the above illustrated example scenario, as in most cases, asingle radio network controller, e.g. radio network controller RNC1, isinvolved when communicating with mobile stations. However, in asituation where two radio network controllers, i.e. both a serving radionetwork controller and a drift radio network controller is involved insupporting communication with mobile stations, some complications arisedue to the functional split between the two radio network controllers.The serving radio network controller is responsible for issuingmeasurement orders to mobile stations and ordering handovers, while thedrift radio network controller is in control of the resources of radiobase stations and cells for which the drift radio network controlleracts as controlling radio network controller. The serving radio networkcontroller and the drift radio network controller interacts over the socalled Iur interface. When ordering the drift radio network controllerto establish radio communication in cells under the control of the driftradio network controller by issuing a Radio Link Setup Request or RadioLink Addition Request message from the serving radio network controllerto the drift radio network controller, the current Radio Link SetupResponse and Radio Link Addition Response messages returned asacknowledgment from the drift radio network controller to the servingradio network controller includes information on which cells constituteneighbor cells to the cells in which communication have beenestablished. In order to enable the serving radio network controller totake account of ranking of neighbor cell sets associated with cellsunder the control of the drift radio network controller, the servingradio network controller needs to be provided with informationindicating said ranking. A simple way of providing the rankinginformation from the drift radio network controller to the serving radionetwork controller would be to require the drift radio networkcontroller to list said neighbor cells in ranking order in the RadioLink Setup Response and Radio Link Addition Response messagesrespectively. Another problem is that it may be difficult for the driftradio network controller to ensure that usage counters of neighbor cellset members are stepped each time handovers are initiated by the servingradio network controller and affecting said neighbor cell set members.This problem may of course e.g. be solved by providing signals from theserving radio network controller to the drift network controllerindicating when handovers affecting said cell members are ordered.Another alternative, would be to simply accept that not all handoversare registered for neighbor cell members on the border between theserving and the drift radio network controller.

[0087] In yet another alternative embodiment of the invention, anoperation and maintenance node, e.g. a Radio Access Network OperationSupport (RANOS) node from Ericsson, may be responsible for gatheringhandover statistics of neighbor cell sets and establishing ranking forcell members of said neighbor cell sets. In this alternative embodiment,the radio network controller, or plural radio network controllers, of aradio access network, would report all handover attempts and/orsuccessful handovers to the operation and maintenance node, indicatingwhich cells were involved in the original active set and which cellswere the target for handover. Using this information, the operation andmaintenance node could assemble statistics on how often neighbor cellsassociated with a certain cell are involved in handovers from activesets including at least said certain cell. The operation and maintenancenode could perform ranking of neighbor cell sets based on theestablished handover statistics and update the radio network controller,or plural radio network controllers, of the ranking of neighbor cellsets associated with cells under the control of respective radio networkcontroller. Preferrably, in order to reduce the load on the interfacebetween radio network controllers and the operation and maintenancenode, each radio network controller gathers information on a pluralityof performed handovers before reporting said handovers to the operationand maintenance node. This alternative embodiment resolves the problemof updating handover usage counters for neighbor cell sets on the borderbetween two radio network controllers.

[0088] According to still another embodiment of the invention, mobilestations could be ordered to perform downlink measurements on monitoredsets including random selections of the neighbor cells associated withcells in the respective active set for a defined statistics gatheringperiod. During this statistics gathering period, handover statisticscould be registered for each neighbor cell as previously described. Uponcompletion of the statistics gathering period, the neighbor cells couldbe ranked based upon the registered handover statistics after whichmobile stations could be ordered to perform downlink measurements onmonitored sets including neighbor cells selected based on said ranking.

[0089] In embodiments of the invention implemented in cellular radiocommunication networks, such as GSM networks, not supportingmacrodiversity/soft handover, the active set supporting communicationbetween the network and a mobile station always includes one cell, i.e.the serving cell.

[0090] As a person skilled in the art appreciates, application of theinvention is in no way limited to only cellular radio communicationnetworks conforming to the UMTS specifications. The invention isgenerally applicable to all cellular systems in which the cellular radiocommunication network orders mobile stations to perform downlinkmeasurements on transmissions in neighbor cells to the cells currentlyserving said mobile stations. Thus, the invention is also applicable incellular radio communication networks adhering to e.g. GSM, PDC,TIA/EIA-136 and CDMA2000 specifications.

1. A method in a cellular radio communication network (NET1) for rankingcell members of a set of neighbor cells (C12-C18, C31-C38, C43, C51,C54-C58) registered in the cellular radio communication network (NET1)as neighbor cells to a first cell (C11) of the cellular radiocommunication network (NET1), the method comprising the steps of:registering (301) for each cell member in said neighbor cell set,handover statistics reflecting how frequent said cell member is involvedin handovers aiming at modifying active sets including at least saidfirst cell to include said cell member; ranking (302) the cell membersof said set of neighbor cells according to a predetermined rule based atleast in part on the registered handover statistics for the differentcell members in the set of neighbor cells.
 2. A method according toclaim 1, wherein prior to performing the ranking step, handoverstatistics are registered for a defined statistics gathering period. 3.A method according to claim 2, wherein each cell member in the set ofneighbor cells is associated with an individual usage counter (406) andsaid registering step is performed by, for the defined statisticsgathering period, incrementing the respective individual usage countersupon involving the corresponding cell members in handovers aiming atmodifying active sets including at least said first cell to include saidcorresponding cell members.
 4. A method according to claim 2, whereinthe statistics gathering period lasts for a predetermined time period.5. A method according to claim 3, wherein the statistics gatheringperiod lasts until the total number of increments of all usage countersduring the statistics gathering period exceeds a predeterminedthreshold.
 6. A method according to claim 3, wherein the length of thestatistics gathering period is defined by a combination of an absolutetime period and the total number of increments of all usage countersduring the statistics gathering period.
 7. A method according to claim3, wherein the statistics gathering period lasts until both the totalnumber of increments of all usage counters during the statisticsgathering period exceeds a predetermined threshold and at least apredetermined minimum time period has passed since the statisticsgathering period begun.
 8. A method according to claim 1, wherein onlysuccessful handovers are included in the handover statistics for eachcell member.
 9. A method according to claim 1, wherein handoverattempts, regardless of whether the handover was successful or not, areincluded in the handover statistics for each cell member.
 10. A methodaccording to claim 1, wherein the registering and ranking steps areperformed repeatedly.
 11. A method according to claim 1, whereinaccording to said predetermined rule, cell members for which theregistered handover statistics indicate frequent involvement inhandovers are generally ranked higher than cell members for which theregistered handover statistics indicate less frequent involvement inhandovers.
 12. A method according to claim 10, wherein according to saidpredetermined rule, cell members for which the registered handoverstatistics indicate frequent involvement in handovers are generallyranked higher than cell members for which the registered handoverstatistics indicate less frequent involvement in handovers while in atleast some of the repeated ranking instances, at least a first cellmember is ranked higher than at least one other cell member morefrequently involved in handovers than the at least first cell member.13. A method according to claim 12, wherein the at least first cellmember is a single cell member which according to the registeredhandover statistics is least frequently involved in handovers andwherein said single cell member is assigned the highest rank.
 14. Amethod according to claim 1, wherein the set of neighbor cells isorganized into at least two subsets of neighbor cells and wherein whenperforming said ranking, each subset is ranked separately.
 15. A methodaccording to claim 14, wherein said at least two subsets include anycombination of: a subset of neighbor cells in the same frequency band asthe first cell; a subset of neighbor cells in a different frequency bandthan the first cell; a subset of neighbor cells in which a radio accesstechnology different from the radio access technology of the first cellis used.
 16. A method according to claim 14, wherein the first cell(C11) is a cell in which radio communication occurs using a CodeDivision Multiple Access based technology and wherein said at least twosubsets include any combination of: a subset (C12-C18) of neighbor cellsusing the same downlink frequency as the first cell (C11); a subset(C31-C38, C43) of neighbor cells using downlink frequencies differentfrom that of the first cell (C11); a subset (C51, C545-C58) of neighborcells in which a radio access technology different from the radio accesstechnology of the first cell (C11) is used.
 17. A method in a cellularradio communication network (NET1) for controlling measurementsperformed by a mobile station (MS1) operating within an area served bythe cellular radio communication network (NET1), the method comprisingthe steps of: determining (311) a current active set (C11-C12) of one ormore cells providing radio communication between the mobile station(MS1) and the cellular radio communication network (NET1); selecting(312) neighbor cells to the cells in the current active set forinclusion in a monitored set of cells; ordering (313) the mobile station(MS1) to perform measurements on downlink transmissions in the cells ofthe monitored set, characterized in that neighbor cells of the cells inthe current active set are ranked using a method according to any one ofclaims 1-16 and that said selection of neighbor cells for inclusion inthe monitored set is based at least in part on said ranking.
 18. Anapparatus (RNC1) in a cellular radio communication network (NET1) forranking cell members of a set of neighbor cells registered in thecellular radio communication network (NET1) as neighbor cells (C12-C18,C31-C38, C43, C51, C54-C58) to a first cell (C11) of the cellular radiocommunication network (NET1), the apparatus including: handoverstatistics gathering circuitry (CP1) adapted to register for each cellmember in the set of neighbor cells, handover statistics reflecting howfrequent said cell member is involved in handovers aiming at modifyingactive sets including at least said first cell to include said cellmember; ranking circuitry (CP1) adapted to rank the cell members of saidset of neighbor cells according to a predetermined rule based at leastin part on the registered handover statistics for the different cellmembers in the set of neighbor cells.
 19. An apparatus according toclaim 18, wherein prior to the ranking circuitry ranking the cellmembers of the neighbor cells, the handover statistics gatheringcircuitry registers handover statistics for a defined statisticsgathering period.
 20. An apparatus according to claim 19, wherein thehandover statistics gathering circuitry implements an individual usagecounter for each cell member in the set of neighbor cells and thehandover statistics gathering circuitry is adapted to, for the definedstatistics gathering period, increment the respective individual usagecounters upon detecting the corresponding cell members being involved inhandovers aiming at modifying active sets including at least said firstcell to include said corresponding cell members.
 21. An apparatusaccording to claim 19, wherein the statistics gathering period lasts fora predetermined time period.
 22. An apparatus according to claim 20,wherein the statistics gathering period lasts until the total number ofincrements of all usage counters during the statistics gathering periodexceeds a predetermined threshold.
 23. An apparatus according to claim20, wherein the length of the statistics gathering period is defined bya combination of an absolute time period and the total number ofincrements of all usage counters during the statistics gathering period.24. An apparatus according to claim 20, wherein the statistics gatheringperiod lasts until both the total number of increments of all usagecounters during the statistics gathering period exceeds a predeterminedthreshold and at least a predetermined minimum time period has passedsince the statistics gathering period begun.
 25. An apparatus accordingto claim 18, wherein the handover statistics gathering circuitry isadapted to include only successful handovers in the handover statisticsfor each cell member.
 26. An apparatus according to claim 18, whereinthe handover statistics gathering circuitry is adapted to includehandover attempts, regardless of whether the handover was successful ornot, in the handover statistics for each cell member.
 27. An apparatusaccording to according to claim 18, wherein according to saidpredetermined rule, cell members for which the registered handoverstatistics indicate frequent involvement in handovers are generallyranked higher than cell members for which the registered handoverstatistics indicate less frequent involvement in handovers.
 28. Anapparatus according to claim 18, wherein the set of neighbor cells isorganized into at least two subsets of neighbor cells and wherein theranking circuitry is adapted to rank each subset separately.
 29. Acellular radio communication network (NET1) including an apparatusaccording to claim 18